This invention relates to a wrist mechanism of an industrial robot, and more particularly to a wrist mechanism for holding a paint spray gun or a work tool thereon, an elongated member such as a hose or a bundle of wires being passed through the mechanism.
U.S. Pat. No. 4,708,580 to Akeel discloses a wrist mechanism of a robot having three independent rotational axes with two points of mutual intersection which are offset from each other. The mechanism has first and second housings, the first housing being adapted to be mounted to the arm of a robot to rotate independently. The second housing supports a tool support member and is capable of independent rotation relative to the first housing via gear trains. These two housings define therein a continuous unobstructed passage through which a flexible paint supply hose or a bundle of power supply wires is passed.
As will be described in more detail hereinafter with reference to the drawing, this prior art construction has a structure which makes it difficult to provide a relatively large opening through the housings for passing therein a flexible hose or a bundle of wires from the outside. In the case of spray painting, such opening is useful for reducing the distance between a spray gun and a color changeover valve. If the flexible paint hose can be led through such opening into the wrist mechanism housing to be coupled to the spray gun mounted on the distal end of the wrist mechanism, it will be possible to mount the color changeover valve in the neighborhood of the wrist mechanism so that the distance from the color changeover valve to the spray gun may be reduced. On the other hand, if a relatively large opening cannot be provided in the housing, it will not be possible to lead the hose into the wrist mechanism so that the hose will have to be passed through a relatively long robot arm and then into and through the wrist mechanism. This means that the color changeover valve will have to be mounted at a proximal end of the robot arm so that there will be a considerable increase of the distance between the color changeover valve and the spray gun. This increase in the distance is not desirable because the length of the hose extending from the color changeover valve increases so that whenever the paint color is changed the increased length of the hose will have to be rinsed or cleaned of the paint with resultant increased length of time required for cleaning.
Furthermore, as will be described later, the axes of rotation of the robot arm and the spray gun are offset due to the structure of the wrist mechanism, a cylindrical dead space is formed in which the spray coating cannot be performed reliably, as will be described in detail hereinafter. Moreover, in the state wherein the axes of the robot arm and the spray gun are maintained parallel with each other, it is not possible to displace the spray gun toward the axis of the robot arm while maintaining the attitude of the spray gun, as will be described in detail hereinafter.